1. Field of the Invention
The present invention relates to an imaging apparatus configured to control an operation of a flash emitting unit which emits flash to an object, and also relates to a control method therefor.
2. Description of the Related Art
There have been conventional imaging apparatuses configured to perform a flash control method for calculating an amount of main flash of a flash unit by performing, just before the main flash, a pre-flash to determine the amount of main flash.
In an imaging apparatus employing an image sensor, such as a complementary metal-oxide semiconductor (CMOS) sensor, of XY address type having pixels arranged in a two-dimensional matrix array on a light receiving surface, starting timings of exposure times respectively corresponding to reading lines of the image sensor vary with the reading lines. Consequently, it is difficult to allow the same reflected flash to fall on the entire light receiving surface of the image sensor. Accordingly, a conventional imaging apparatus performs a flash control method, according to which a pre-flash is synchronized with the starting timing of the exposure time corresponding to each reading line, thereby to continuously perform pre-flashes the number of which is equal to that of the reading lines (see Japanese Patent Application Laid-Open No. 2002-359774).
According to a flash control method using a global shutter in an imaging apparatus which employs an image sensor of the XY address type, a pre-flash of a flash unit is performed by adjusting the starting timings of exposure times respectively corresponding to all of the reading lines to a specific timing with the global shutter. However, in a case where the number of reading lines is large, the exposure time corresponding to the last reading line is long as compared with the exposure time corresponding to the first reading line. Thus, there is a large difference in exposure time between the first reading line and the last reading line. Accordingly, a lot of external light other than the flash is received by the last half of the reading lines of the image sensor.
Thus, a conventional imaging apparatus reduces an exposure time required to read data from the entire light receiving surface of the image sensor by thinning out the reading lines to be used, that is, decreasing the number of lines used for a pre-flash, so as to reduce a light exposure due to external light other than the flash. In this case, the conventional imaging apparatus performs a flash control method according to which an amount of main flash is calculated according to a difference between luminance data obtained by performing a pre-flash and luminance data obtained by similarly driving all pixels of the flash unit without performing a pre-flash (see Japanese Patent Application Laid-Open No. 2006-50337).
A conventional imaging apparatus, such as a digital video camera, employing a CMOS sensor performs a flash control method according to which an amount of main flash is calculated by comparing luminance data, which is obtained by a pre-flash performed immediately before a main flash, and luminance data obtained when no pre-flash is performed.
In a case where an electronic shutter speed is low, there is an overlapping timing with which a pre-flash is performed so that the exposure times respectively corresponding to the reading lines of the image sensor overlap. Therefore, when a pre-flash is performed at this pre-flash timing, the same reflected flash can fall on all of the reading lines. However, in a case where the electronic shutter speed is high, there is no overlapping timing with which a pre-flash is performed so that the exposure times respectively corresponding to the reading lines of the image sensor overlap. Accordingly, the same reflected flash cannot fall on all of the reading lines.
Thus, a conventional imaging apparatus employs a flash control method according to which a pre-flash is synchronized with the starting timing of the exposure times respectively corresponding to each of the reading lines, and according to which pre-flashes of the number of times equal to the number of the reading lines are consecutively performed to enable reflected flash to fall on the entire light receiving surface. Alternatively, according to another flash control method, the starting timings of exposure times respectively corresponding to all of the reading lines are adjusted to a specific timing by a global shutter to make the exposure times to overlap, so that the overlapping timing is generated. However, according to these methods, in a case where the number of reading lines is large, the exposure time corresponding to the last reading line is long as compared with the exposure time corresponding to the first reading line. Thus, there is a large difference in exposure time between the first reading line and the last reading line.
To prevent occurrence of a large difference in exposure time, a flash control method has been developed, according to which an exposure time required to read data from the entire light receiving surface of the image sensor is reduced by thinning out the reading lines to be used, that is, decreasing the number of lines used for a pre-flash, so that the same reflected flash can fall on the entire light receiving surface.
According to the former method for adjusting the starting timings of exposure times to be the same, it is necessary to divide a pre-flash by a number which is equal to the number of reading lines of the image sensor and to continuously emit flash. However, in this case, it is necessary to set an amount of flash emitted for a single emission time to be equal to or more than a value predetermined according to specifications of a flash lamp of the flash unit. Consequently, the former method has a drawback in that a resolution obtained during a pre-flash is low. Also, according to the latter method for thinning out the reading lines used for a pre-flash, the number of reading lines used for a pre-flash is reduced so as to suppress the influence of a light exposure of external light other than pre-flash on the entire light receiving surface. Consequently, the latter method also has a drawback in that a resolution obtained during a pre-flash is low.
Accordingly, in the conventional imaging apparatuses, in a case where the electronic shutter speed is equal to or higher than a predetermined value, it is difficult to accurately calculate an amount of main flash of a flash unit.